A recurring situation: the prototype runs hotter than the budget allows, the engineer goes back to the aluminum PCB, and moves the dielectric from a standard grade to a higher-conductivity grade. On the datasheet the conductivity figure more than doubles. The rebuilt unit does run cooler, but far less than the datasheet movement suggested. Sometimes the improvement is ten or twenty percent. Sometimes it is barely measurable. The conclusion that gets written down is usually "the supplier's material does not match its datasheet."
That conclusion is almost always wrong. The datasheet states the bulk conductivity of one material layer. Temperature rise is the result of an entire thermal resistance chain. These are not quantities at the same level. Only one segment of the chain changed; everything else stayed exactly as it was. The overall improvement was never going to equal the improvement in that one segment.
The engineering consequence matters more than the disappointment. If the dielectric was not the dominant segment to begin with, money spent on it buys an improvement that the other segments dilute. To know where the money should go, the chain has to be taken apart.
1. Heat crosses five segments, not one
For a power device mounted on an aluminum PCB, heat leaving the die passes through these interfaces and materials in sequence:
- Inside the device, from junction to the bottom pad. Set by the device manufacturer and stated in the device datasheet. At selection time you can choose a different device; you cannot change this segment.
- The solder joint and mounting interface. The solder layer between the device bottom and the board's copper. Solder itself conducts well, but the real resistance of this segment depends heavily on whether the fill is complete. Voids under a bottom thermal pad force the heat flow to detour.
- Lateral spreading in the copper. Heat entering the copper does not travel straight down. It spreads across the copper plane first, then passes down through the dielectric. How far it spreads determines the effective cross-section available to the next segment.
- The dielectric. This is the segment the datasheet conductivity figure describes. It is the one segment in the chain whose material conducts markedly worse than metal, which is why it gets treated as the bottleneck. Sometimes it genuinely is.
- The metal base, and the interface between board and heatsink. The aluminum base itself conducts well and usually contributes little. But the layer between the board's back face and the heatsink — thermal grease, a gap pad, or direct clamping — is set entirely by assembly, and it varies enormously.
Putting the five in series makes two things immediately clear.
First, the dielectric is one segment out of five. Even if it accounts for a third of the whole chain, halving it improves the chain by roughly a sixth. That is the arithmetic behind "the datasheet doubled and the temperature dropped fifteen percent." It is not mis-specification. It is how series resistances behave.
Second, three of the five segments lie outside material selection. Segment 1 belongs to the device manufacturer, segment 2 to the assembly process, and the second half of segment 5 to final mechanical assembly. Material selection reaches segments 3 and 4, and the first half of segment 5.
2. Which segment dominates depends on what the board looks like
There is no universal answer to "is the dielectric the bottleneck." It depends on the design, and several indicators can be read without instrumentation:
- Small heat source area with high power density raises the weight of segment 3. The heat is forced downward before it has spread, so the dielectric sees a small effective area. Here a higher-conductivity dielectric does help noticeably — but so does thicker copper or a larger thermal copper area, often by a similar margin and at lower cost.
- Large heat source area at moderate power density lowers the dielectric's share of the total, and segment 5 frequently overtakes it. These are the designs where upgrading the dielectric most often produces spend without effect.
- Devices with bottom thermal pads carry the largest uncertainty in segment 2. If fill under the pad is inconsistent, board-to-board variation in temperature rise can exceed the entire benefit of a dielectric upgrade. In other words, a process consistency problem will mask a material improvement completely.
- Assemblies that export heat through a metal enclosure or heatsink should have segment 5 checked first in nearly every case. It is the one segment that lies outside the PCB supplier's control and is frequently the largest.
The practical ordering follows from this: rule out segments 2 and 5 before discussing upgrades to 3 and 4. The reason is cost, not theory. The first two are assembly and process questions, and eliminating them requires no BOM change and no new prototype run. A material upgrade means a new stackup, new pricing and new qualification. Do the cheap half first.
3. The segments fabrication decides are the ones you cannot see
Two segments have real-world behaviour that is settled during fabrication, and the datasheet does not describe either of them.
How well the dielectric actually bonded. The datasheet figure assumes this layer is continuous, dense and fully in contact with the surfaces above and below it. If lamination leaves localised gaps, the real resistance of this segment is higher than calculated, and unevenly so — a gap directly beneath the heat source and a gap near the board edge have entirely different consequences. The mechanism is the same family as voiding and delamination in multilayer pressing, and the control approach is covered in lamination void and delamination prevention.
The copper-to-dielectric interface bond. Its direct contribution to thermal resistance is small, but it determines whether the previous segment holds up over time. Boards with a weak interface bond degrade across thermal cycling: resistance is within budget at shipment, and temperature rise climbs after a period in service.
What these two share is this: neither can be judged from the appearance of a finished board, and neither is solved by tightening incoming inspection. They can only be traced through process evidence. The actionable move for an engineer is not to request a thermal resistance measurement, but to state the thermally critical region in the order — which location sits under the main heat source, and that localised gaps are not acceptable there — so the board is managed as a thermally critical part rather than a general aluminum PCB. SKPCB runs multiple in-process inspection gates (Q-GATES) and accepts to the IPC-6012 framework (QC-IPC). It is worth being precise about what that means: those gates answer "this board conforms to the acceptance requirements," which is not the same statement as "this board's thermal resistance equals your calculated value." The second one has to be verified on your own assembly.
4. What to specify, and what not to expect
If the conclusion is that the dielectric genuinely should be upgraded, what goes to the supplier should include:
- The location and extent of the thermally critical region, not just "high thermal conductivity aluminum PCB." The supplier needs to know which area must not carry localised defects.
- The design intent of the thermal copper area. Lateral spreading in segment 3 depends on copper layout. If the thermal copper gets carved up to route signals, part of the material upgrade is given back. This belongs in the design phase, not in material selection as a remedy.
- Operating temperature range and thermal cycling conditions. These bear on long-term interface behaviour, which is a different question from a one-time temperature rise measurement.
- How the back face mounts to the heatsink. This sets segment 5. The PCB supplier does not control it, but it will determine the temperature you finally observe, and it also drives the flatness requirement on the board.
Equally important is what not to expect. Do not expect the ratio of datasheet conductivities to equal the ratio of temperature improvements — the five series segments above explain why. And do not use a single-point temperature reading to decide which segment is at fault. The same temperature rise can arise from five different causes, and without separating them there is no attribution.
If the dielectric upgrade path is exhausted and the thermal budget still is not met, the question is no longer about the dielectric; it becomes a base material or board type decision. For choosing between aluminum and copper base, see the manufacturing criteria for metal core base selection. For the three-way trade among dielectric conductivity, withstand voltage and thinness, and how to put it on a drawing, see the aluminum PCB dielectric trade-off. If the requirement has moved beyond what a metal base can deliver, the next step is a ceramic substrate, with process routes covered in ceramic substrate material and process selection. Machining limits on aluminum boards are a separate set of constraints, covered in aluminum PCB machining and profiling limits. The product entry point is Aluminum PCB, and the quality framework is described under Quality.
FAQ
Q: The datasheet conductivity doubled. How much should temperature rise fall?
There is no ratio that can be applied. Temperature rise is set by the whole chain from junction to heatsink, and the dielectric is one segment of it. If that segment was a third of the total, halving it improves the chain by about a sixth; if it was a fifth, the improvement is smaller still. Estimating the benefit requires knowing that segment's share in your specific board, which depends on heat source area, copper layout and the assembly interface — not on the material datasheet.
Q: How do I find which segment dominates in my design?
Work by elimination, cheapest first. Check the assembly interface (segment 5) first, since changing an interface material or re-clamping costs the least. Then check fill consistency at bottom-pad devices (segment 2) by looking at whether temperature rise is scattered across boards from the same batch. If both are ruled out and the heat source is small with high power density, attention moves to copper spreading and the dielectric (segments 3 and 4). The ordering exists because the first two require no BOM change and no new prototype.
Q: Boards from the same batch show clearly different temperature rise. Is that a material batch problem?
Batch variation is one possible cause but usually not the first suspect. More commonly it is solder fill consistency at the mounting interface, or clamping consistency at the heatsink interface. Both produce scatter within a batch, whereas material batch variation tends to show up between batches rather than inside one. The diagnostic question is whether the scatter is within-batch or batch-to-batch.
Q: Can I ask the PCB supplier for the thermal resistance of this board?
Treat such a number carefully. A PCB supplier can control and evidence that materials and processes conform to what was agreed. Thermal resistance is a property of the entire chain and depends on the device, the solder joint, copper layout and final assembly, most of which lie outside the PCB supplier's boundary. The more reliable approach is to write the thermally critical region and the non-acceptance of defects there into the order requirements, then verify actual temperature rise on your own assembly.
Q: Thicker copper or a better dielectric — which is worth doing?
It depends on the power density of the heat source. With a small, dense heat source, heat is forced downward before it has spread across the copper, so added copper thickness or a larger thermal copper area increases the effective cross-section of the next segment, often giving a benefit comparable to a dielectric upgrade at lower cost. When the heat source is already large, lateral spreading is not the bottleneck and thicker copper returns little. Note that copper thickness also affects routing and etching, so it is not purely a thermal decision.